A frequently employed technique for eliminating visual defects of the human eye—such as, for example, myopia or hyperopia or astigmatism—is so-called LASIK. LASIK stands for laser in-situ keratomileusis and designates a technique in which firstly a small cover disc in the cornea is cut free which is folded aside in order to expose the underlying tissue regions of the cornea. These exposed tissue regions are then treated in ablating manner by means of focused UV laser radiation—i.e. corneal material is removed in accordance with an ablation profile ascertained individually for the patient. The small cover disc is usually designated in specialist circles as a flap and is not severed completely from the remaining tissue of the cornea but is still connected to the rest of corneal tissue in a hinge region which in specialist circles is generally designated as a hinge. This enables a simple folding-away of the flap and, above all, a simple folding-back of the flap after the ablation. On account of the removal of material, a changed shape of the anterior surface of the cornea arises after the flap has been folded back. The associated result of this is a different refractive behaviour of the cornea and consequently of the overall system constituted by the eye. By suitable definition of the ablation profile it can be ensured that the visual defect is at least distinctly attenuated and, at best, is almost completely eliminated.
Various procedures for the preparation of the flap are known in the state of the art. One procedure uses a mechanical microkeratome—i.e. a microsurgical scalpel which cuts into the cornea with a cutting blade which is ordinarily driven in oscillating manner. Another procedure, which will be considered in more detail within the scope of the invention, uses focused short-pulse laser radiation for the purpose of preparing the flap. In this case, laser radiation with pulse durations within the femtosecond range, for example within the low three-digit femtosecond range, is ordinarily employed. In addition, the laser radiation usually has a wavelength above about 300 nm, in order to enable a coupling of the radiant energy deep into the corneal tissue. LASIK treatments in which the flap is prepared by means of such short-pulse laser radiation are often designated as fs LASIK.
For the generation of incisions by means of focused laser radiation in transparent material (transparent to the laser radiation), the so-called laser-induced optical breakthrough is utilised as a physical effect. This breakthrough ultimately results in a photodisruption of the irradiated tissue in the region of the focus. The laser radiation that is beamed in brings about a local vaporisation of the irradiated material at the focal point. In the process, gases arise which—to the extent that they are not conducted away to the outside—collect in internal cavities or are absorbed by the adjoining material. It has been found that in the course of LASIK treatments of the human eye a residence in the cornea of the gases arising in the course of preparation of the flap can lead to problems in the course of the subsequent laser ablation. In particular, it has been found that these gases can render difficult a precise tracking of the eye by means of an eye-tracker. Laser systems that are employed for the ablation of corneal tissue frequently possess such an eye-tracker, in order to register eye movements during the laser treatment and to reposition the laser radiation correspondingly. As a rule, the eye-trackers are constructed from a camera and suitable image-evaluation software which evaluates the images recorded by the camera and detects changes in the position of the eye. Frequently the image-evaluation software evaluates characteristic features of the eye—for instance, defined points of the iris or/and the pupillary centre or/and the corneal apex or/and the limbus. It has been shown that accumulations of gas remaining in the cornea, which have arisen in the course of preparation of the flap, can impede the acquisition of such characteristic features of the eye. It goes without saying that for the success of the operation a precise functioning of the eye-tracker is absolutely essential.